jfs_debugfs

Interaction with JFS debugfs entries is performed via standard shell commands after ensuring the debugfs filesystem is mounted.

To mount debugfs (if not already mounted):
mount -t debugfs none /sys/kernel/debug

To list available JFS debug entries for a specific device (e.g., sdb1):
ls /sys/kernel/debug/jfs/sdb1/

To read a debug entry (e.g., dirty_pages):
cat /sys/kernel/debug/jfs/sdb1/dirty_pages

To modify a tunable debug entry (e.g., max_dirty_pages):
echo new_value > /sys/kernel/debug/jfs/sdb1/max_dirty_pages

The term 'jfs_debugfs' refers to the JFS (Journaled File System) specific entries exposed through the Linux kernel's debugfs virtual filesystem. Unlike a standalone command with its own set of options, 'jfs_debugfs' is not a dedicated executable. Instead, it represents the interface for inspecting and sometimes modifying internal JFS parameters and statistics at runtime.

Administrators and developers can interact with these debugfs entries by navigating the virtual directory structure, typically located under /sys/kernel/debug/jfs/, and using standard shell commands such as cat to read values, echo to write values (for tunable parameters), and ls to list available entries. This mechanism provides a low-level view into the JFS module's operational state, aiding in debugging, performance analysis, and understanding filesystem behavior.

Interacting with kernel debugfs entries, including those for JFS, requires root privileges. Modifying values in debugfs can have significant and potentially unstable impacts on system behavior, filesystem integrity, or performance. This interface is primarily intended for kernel developers and advanced users for debugging purposes. It is not a stable API and its structure or available entries may change between kernel versions. Users should exercise extreme caution and fully understand the implications of any changes made via this interface.

The exact paths and available entries under /sys/kernel/debug/jfs/device_name/ can vary depending on the kernel version and specific JFS module. However, common types of information found include:

• Statistics: Counters related to I/O operations, transaction commits, inode usage, and buffer management.
• Tunable Parameters: Settings that can influence JFS behavior, such as dirty page thresholds or transaction logging verbosity.
• Internal State: Details about the filesystem's internal data structures, such as transaction logs, inode maps, or allocation bitmaps, often presented in a human-readable format for diagnostic purposes.

JFS (Journaled File System), originally developed by IBM for OS/2 and AIX, was introduced into the Linux kernel around version 2.4.20. The debugfs mechanism itself was added to the Linux kernel to provide a simple way for kernel developers to export debugging information and control parameters. The JFS module, like many other kernel components, integrated its internal debugging interface with debugfs to allow for runtime inspection and diagnostics, making the 'jfs_debugfs' interface a part of the broader kernel debugging ecosystem.

debugfs(5), mount(8), cat(1), echo(1), ls(1), jfs_fsck(8), jfs_tune(8)